A grayscale voltage generating circuit for a liquid crystal display device of this type has been disclosed in the specification of Japanese Patent Kokai Publication JP-A-6-348235, by way of example. Specifically, the specification proposes a grayscale voltage generating circuit comprising a plurality of fixed resistors serially connected between a high-potential reference voltage and a low-potential reference voltage, and voltage varying means capable of varying the voltages at the nodes between the fixed resistors between the high-potential reference voltage and the low-potential reference voltage, with the voltages at the nodes between the fixed resistors being used as grayscale signals.
FIG. 5 illustrates part of this prior-art grayscale voltage generating circuit. Though there are eight outputs of positive polarity and eight outputs of negative polarity according to the description given in the above-mentioned Japanese Patent Kokai Publication JP-A-6-348235 (see FIG. 1 of the specification), here the grayscale voltage generating circuit will be described as a circuit having three outputs of each of the positive and negative polarities.
As shown in FIG. 5, the prior-art grayscale voltage generating circuit includes fixed resistors R1 to R4 and a variable resistor VR3 serially connected between a reference voltage VH and a reference voltage VL, variable resistors VR1, VR2 serially connected between the reference voltage VH and reference voltage VL, an amplifier circuit (a voltage-follower operational amplifier) A1 connected between the node of the fixed resistors R1, R2 and the variable resistor VR1, and an amplifier circuit A2 connected between the node of the fixed resistors R3, R4 and the variable resistor VR2.
The reference voltages VH and VL in this grayscale voltage generating circuit are output as is as a high-potential grayscale voltage V0H on the side of positive polarity and a high-potential grayscale voltage V0L on the side of negative polarity, respectively. A halftone grayscale voltage V1H and a low-potential grayscale voltage V2H on the side of positive polarity as well as a low-potential grayscale voltage V2L and a halftone grayscale voltage V1L on the side of negative polarity can be adjusted by adjusting the resistance values of the variable resistors VR1, VR2 connected to the input side of the amplifier circuits A1, A2, respectively, and the resistance value of the variable resistor VR3 located between the grayscale voltage V2H and grayscale voltage V2L.
The above-described grayscale voltage generating circuit makes it possible to improve upon a change in grayscale characteristic brought about by a change in viewing angle, namely the angle from which a liquid crystal display monitor is viewed. This change in grayscale characteristic due to a change in viewing angle is one characteristic of a liquid crystal display monitor primarily of the twisted nematic type.
Owing to the characteristics of the liquid crystal in a liquid crystal monitor, it is required that identical positive and negative voltages be applied in AC drive. Furthermore, it is necessary to correct for a shift (referred to as a "feed-through characteristic") in ground potential of the liquid crystal caused by a difference in the voltages applied to the liquid crystal (e.g., as when 1 V is applied and when 5 V is applied). This correction is referred to as a "feed-through correction."
In the above-described grayscale voltage generating circuit, feed-through correction values are decided by the potential dividing ratio of the serially connected fixed resistors R2, R3 and variable resistor VR3 and the adjustment of the resistance values of the variable resistors VR1, VR2. Consequently, even if the variable resistors VR1, VR2 are varied to obtain the optimum grayscale characteristic and optimize the feed-through correction values of the grayscale voltages V1H, V1L in order to improve upon the grayscale characteristic based upon the viewing angle, the feed-through correction values of the low-potential grayscale voltages V2H, V2L are decided solely by the potential dividing ratio determined by the fixed resistors R2, R3 and variable resistor VR3 between the halftone-level grayscale voltages V1H, V1L. In other words, adjusting the variable resistor VR3 merely changes the potential difference between the low-potential grayscale voltages V2H and V2L. This means that these feed-through correction values cannot be adjusted individually for each of the positive and negative polarities.
As a consequence of the foregoing, the feed-through correction values of the grayscale voltages V2H, V2L differ from the appropriate values.